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Drilling fluids are, among other things, expected to transport cuttings and control fluid loss. The rheology of drilling fluids plays an important roll in both of these aspects of drilling fluid performance. A number of years ago the American Petroleum Institute established a set of standards for evaluating the rheology of drilling fluids. While drilling fluid technology has evolved and drilling fluids have become more rheologically complex, the standards have not changed sufficiently to adequately characterize the new generation of drilling fluids.


The American Petroleum Institute (API) has established a set of standards for the rheological characterization of drilling fluids, API BUL 13D. A second edition was published in 1985 and the third edition is currently in the last stages of the publication process. API 13D covers everything from basic rheological concepts to data acquisition and analysis. It has a good discussion of non-Newtonian fluid models but this discussion does not include adequate coverage of fluid- models that describe fluids which exhibit Newtonian behavior at low shear rates. For many years, before the advent of low-solids drilling fluids, the models covered in API 13D were adequate. Environmental and operational considerations have encouraged a gradual shift from traditional drilling fluids to low-solids muds that contain polymers to provide the desirable properties. Changing to polymer-based fluids has brought about changes in the mechanisms for viscosity development and fluid-loss control. These fluids are not adequately characterized by the tests outlined in API 13D.

When sheared, a typical non-Newtonian fluid will exhibit flow behavior similar to that shown in Figure 1. The fluid first gives a Newtonian response (lower Newtonian region) to the shear rate and then transitions into a power law region. When the shear rate gets high enough a second transition occurs to Newtonian behavior (upper Newtonian region). Traditionally in the oilfield, measurements are made in what is thought to be the power law region. As we will see, measurements made with the standard six speed instrument do not always provide points within the power-law region. Both the upper and lower Newtonian regions, typically, have been ignored.

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